Abstract
Abstract A statistical survey of spectral anisotropy of space plasma turbulence is performed using five years of measurements from the Magnetospheric Multiscale mission in the magnetosheath. By measuring the five-point second-order structure functions of the magnetic field, we have for the first time quantified the three-dimensional anisotropies and scalings at sub-ion scales (<100 km). In the local reference frame defined with respect to local mean magnetic field , the “statistical eddies” are found to be mostly elongated along and shortened in the direction perpendicular to both and local field fluctuations. From several d i (ion inertial length) toward ∼0.05 d i , the ratio between eddies’ parallel and perpendicular lengths features a trend of rise then fall, whereas the anisotropy in the perpendicular plane appears scale-invariant. Specifically, the anisotropy relations for the total magnetic field at 0.1–1.0 d i are obtained as , and , respectively. Our results provide new observational evidence to compare with phenomenological models and numerical simulations, which may help to better understand the nature of kinetic-scale turbulence.
Highlights
The energy distribution at a certain scale is known to be not isotropic in the turbulence of magnetized plasma, known as spectral anisotropy (Cho & Vishniac 2000)
Most of our experimental knowledge of space plasma anisotropy comes from in-situ observations made within the solar wind (SW), which is a nearly collisionless plasmas stream released from the Sun (Bruno & Carbone 2013)
For the SF of the total magnetic field energy as projected in the (l, l⊥2 + L2⊥) plane, the contours of S2(5)(l; B) are elongated in the parallel directions (Figure 2a), where the values at perpendicular direction are much larger than the ones in the parallel direction (i.e. S2(5)(l; B) at l⊥ = 60 km is more than 100 times larger than the one at l = 60 km)
Summary
The energy distribution at a certain scale (or k space) is known to be not isotropic in the turbulence of magnetized plasma, known as spectral anisotropy (Cho & Vishniac 2000). At large magnetohydrodynamic (MHD) scales, the pattern of correlation function for the magnetic field at 1 AU has two major components referred to as “Maltese cross”, exhibiting elongations in both parallel and perpendicular direction with regard to B0 (Matthaeus et al 1990) This signature is summarized as the “slab+2D” model, which assumes no specific nature of the fluctuations but just describe the fluctuations as a combination of waves with k and structures with k⊥. Using measurements from Magnetospheric Multiscale mission (MMS) (Burch et al 2016), Chen & Boldyrev (2017) studied the two-point SF of the magnetic field in the same plane and provided evidence of strong anisotropy at smaller scales (11 < kρi < 57).
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